Power transfer unit for four-wheel drive vehicle

ABSTRACT

A drive axle assembly includes first and second axleshafts connected to a pair of wheels and a drive mechanism operable to selectively couple a driven input shaft to one or both of the axleshafts. The drive mechanism includes a differential, first and speed changing units and first and second mode clutches. The first mode clutch is operable in association with the first speed changing unit to increase the rotary speed of the first axleshaft which, in turn, causes a corresponding decrease in the rotary speed of the second axleshaft. The second mode clutch is operable in association with the second speed changing unit to increase the rotary speed of the second axleshaft so as to cause a decrease in the rotary speed of the first axleshaft. A control system controls actuation of both mode clutches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/265,219 filed Nov. 02, 2005 which is a continuation of U.S. patentapplication Ser. No. 10/855,904 filed May 27, 2004, now U.S. Pat. No.7,004,876.

FIELD OF THE INVENTION

The present invention relates generally to differential assemblies foruse in motor vehicles and, more specifically, to a differential assemblyequipped with a torque vectoring drive mechanism and an active controlsystem.

BACKGROUND OF THE INVENTION

In view of consumer demand for four-wheel drive vehicles, many differentpower transfer system are currently utilized for directing motive power(“drive torque”) to all four-wheels of the vehicle. A number of currentgeneration four-wheel drive vehicles may be characterized as includingan “adaptive” power transfer system that is operable for automaticallydirecting power to the secondary driveline, without any input from thevehicle operator, when traction is lost at the primary driveline.Typically, such adaptive torque control results from variable engagementof an electrically or hydraulically operated transfer clutch based onthe operating conditions and specific vehicle dynamics detected bysensors associated with an electronic traction control system. Inconventional rear-wheel drive (RWD) vehicles, the transfer clutch istypically installed in a transfer case for automatically transferringdrive torque to the front driveline in response to slip in the reardriveline. Similarly, the transfer clutch can be installed in a powertransfer device, such as a power take-off unit (PTU) or in-line torquecoupling, when used in a front-wheel drive (FWD) vehicle fortransferring drive torque to the rear driveline in response to slip inthe front driveline. Such adaptively-controlled power transfer systemcan also be arranged to limit slip and bias the torque distributionbetween the front and rear drivelines by controlling variable engagementof a transfer clutch that is operably associated with a centerdifferential installed in the transfer case or PTU.

To further enhance the traction and stability characteristics offour-wheel drive vehicles, it is also known to equip such vehicles withbrake-based electronic stability control systems and/or tractiondistributing axle assemblies. Typically, such axle assemblies include adrive mechanism that is operable for adaptively regulating theside-to-side (i.e., left-right) torque and speed characteristics betweena pair of drive wheels. In some instances, a pair of modulatableclutches are used to provide this side-to-side control, as is disclosedin U.S. Pat. Nos. 6,378,677 and 5,699,888. According to an alternativedrive axle arrangement, U.S. Pat. No. 6,520,880 discloses ahydraulically-operated traction distribution assembly. In addition,alternative traction distributing drive axle assemblies are disclosed inU.S. Pat. Nos. 5,370,588, 5,415,598 and 6,213,241.

As part of the ever increasing sophistication of adaptive power transfersystems, greater attention is currently being given to the yaw controland stability enhancement features that can be provided by such tractiondistributing drive axles. Accordingly, this invention is intended toaddress the need to provide design alternatives which improve upon thecurrent technology.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide adrive axle assembly for use in motor vehicles which is equipped with anadaptive yaw control system.

To achieve this objective, a drive axle assembly according to oneembodiment of the present invention includes first and second axleshaftsconnected to a pair of wheels and a torque distributing drive mechanismthat is operable for transferring drive torque from a driven input shaftto the first and second axleshafts. The torque distributing drivemechanism includes a differential, first and second speed changingunits, and first and second mode clutches. The differential includes aninput component driven by the input shaft, a first output componentdriving the first axleshaft and a second output component driving thesecond axleshaft. The first speed changing unit includes a firstplanetary gearset having a first sun gear driven by the first outputcomponent, a first ring gear, and a set of first planet gears rotatablysupported by the input component and which are meshed with the firstring gear and the first sun gear. The second speed changing unitincludes a second planetary gearset having a second sun gear driven bythe second output component, a second ring gear, and a set of secondplanet gears rotatably supported by the input component and which aremeshed with the second ring gear and the second sun gear. The first modeclutch is operable for selectively braking rotation of the first ringgear. Likewise, the second mode clutch is operable for selectivelybraking rotation of the second ring gear. Accordingly, selective controlover actuation of the first and second mode clutches provides adaptivecontrol of the speed differentiation and the torque transferred betweenthe first and second axleshafts. A control system including and ECU andsensors are provided to control actuation of both mode clutches.

In accordance with another embodiment of a drive axle assembly accordingto the present invention, the torque distributing drive mechanismincludes a differential, first and second speed changing units, andfirst and second mode clutches. The differential includes an inputcomponent driven by the input shaft and first and second outputcomponents. The first speed changing unit is a first planetary gearsethaving a first sun gear driving the first axleshaft, a first ring geardriven by the first output component, and a set of first planet gearsrotatably supported by the input component and which are meshed with thefirst sun gear and the first ring gear. The second speed changing unitis a second planetary gearset having a second sun gear driving thesecond axleshaft, a second ring gear driven by the second outputcomponent, and a set of second planet gears rotatably supported by theinput component and which are meshed with the second sun gear and thesecond ring gear. The first mode clutch is again operable forselectively braking rotation of the first ring gear while the secondmode clutch is operable for selectively braking rotation of the secondring gear. The control system controls actuation of the first and secondmode clutches for controlling the speed differentiation and torquetransferred between the first and second axleshafts.

Pursuant to an alternative objective of the present invention, thetorque distributing drive mechanism can be utilized in a power transferunit, such as a transfer case, of a four-wheel drive vehicle toadaptively control the front-rear distribution of drive torque deliveredfrom the powertrain to the front and rear wheels.

Further objectives and advantages of the present invention will becomeapparent by reference to the following detailed description of thepreferred embodiments and the appended claims when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a diagrammatical illustration of an all-wheel drive motorvehicle equipped with a drive axle having a torque distributingdifferential assembly and an active yaw control system according to thepresent invention;

FIG. 2 is a schematic illustration of the torque distributingdifferential assembly shown in FIG. 1;

FIG. 3 is a diagrammatical illustration of the power-operated actuatorsassociated with the torque distributing differential assembly of thepresent invention;

FIGS. 4 and 5 are schematic illustrations of alternative embodiments ofthe torque distributing differential assembly of the present invention;

FIG. 6 is a diagrammatical illustration of the torque distributingdifferential assembly of the present invention installed in a powertransfer unit for use in a four-wheel drive vehicle; and

FIG. 7 is a schematic drawing of the power transfer unit shown in FIG.6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an all-wheel drive vehicle 10 includes an engine 12transversely mounted in a front portion of a vehicle body, atransmission 14 provided integrally with engine 12, a front differential16 which connects transmission 14 to front axleshafts 18L and 18R andleft and right front wheels 20L and 20R, a power transfer unit (“PTU”)22 which connects front differential 16 to a propshaft 24, and a rearaxle assembly 26 having a torque distributing drive mechanism 28 whichconnects propshaft 24 to axleshafts 30L and 30R for driving left andright rear wheels 32L and 32R. As will be detailed, drive mechanism 28is operable in association with a yaw control system 34 for controllingthe transmission of drive torque through axleshafts 30L and 30R to rearwheels 32L and 32R.

In addition to an electronic control unit (ECU) 36, yaw control system34 includes a plurality of sensors for detecting various operational anddynamic characteristics of vehicle 10. For example, a front wheel speedsensor 38 is provided for detecting a front wheel speed value based onrotation of propshaft 24, a pair of rear wheel speed sensors 40 areoperable to detect the individual rear wheel speed values based rotationof left and right axle shafts 30L and 30R, and a steering angle sensor42 is provided to detect the steering angle of a steering wheel 44. Thesensors also include a yaw rate sensor 46 for detecting a yaw rate ofthe body portion of vehicle 10 and a lateral acceleration sensor 48 fordetecting a lateral acceleration of the vehicle body. As will bedetailed, ECU 36 controls operation of a pair of mode clutchesassociated with drive mechanism 28 by utilizing a control strategy thatis based on input signals from the various sensors.

Rear axle assembly 26 includes an axle housing 52 within which drivemechanism 28 is rotatably supported. In general, torque distributingdrive mechanism 28 includes an input shaft 54, a differential 56, afirst or left speed changing unit 58L, a second or right speed changingunit 58R, a first or left mode clutch 60L and a second or right modeclutch 60R. As seen, input shaft 54 includes a pinion gear 64 that is inconstant mesh with a hypoid ring gear 66. Ring gear 66 is fixed forrotation with a carrier 68 associated with differential 56. Differential56 is a bevel gearset that is operable to transfer drive torque fromcarrier 68 to axleshafts 30L and 30R while permitting speeddifferentiation therebetween. Differential 56 includes a first or leftside gear 70L fixed for rotation with left axleshaft 30L, a second orright side gear 70R fixed for rotation with right axleshaft 30R, and atleast one pair of pinion gears 72 rotatably supported on pinion shafts74 that are fixed for rotation with carrier 68.

Left speed changing unit 58L is a planetary gearset having a sun gear76L fixed for rotation with left axleshaft 30L, a ring gear 78L, and aplurality of planet gears 80L rotatably supported on carrier 68 andwhich are meshed with both sun gear 76L and ring gear 78L. Right speedchanging unit 58R is generally identical to left speed changing unit 58Land is shown to include a sun gear 76R fixed for rotation with rightaxleshaft 30R, a ring gear 78R, and a plurality of planet gears 80Rrotatably supported on carrier 68 and meshed with both sun gear 76R andring gear 78R.

With continued reference to FIG. 2, first mode clutch 60L is shown to beoperably disposed between ring gear 78L of first speed changing unit 58Land housing 52. In particular, first mode clutch 60L includes a clutchhub 90L that is connected for common rotation with ring gear 78L and adrum 92L that is non-rotatably fixed to housing 52. As seen, a bearingassembly 88L supports hub 90L for rotation relative to carrier 68. Firstmode clutch 60L also includes a multi-plate clutch pack 94L that isoperably disposed between drum 92L and hub 90L, and a power-operatedclutch actuator 96L. First mode clutch 60L is operable in a first or“released” mode so as to permit unrestricted rotation of ring gear 78L.In contrast, first mode clutch 60L is also operable in a second or“locked” mode to brake rotation of ring gear 78L, thereby causing sungear 76L to be driven at an increased rotary speed relative to carrier68. Thus, first mode clutch 60L functions in its locked mode to increasethe rotary speed of left axleshaft 30L which, in turn, causesdifferential 56 to generate a corresponding decrease in the rotary speedof right axleshaft 30R, thereby directing more drive torque to leftaxleshaft 30L than is transmitted to right axleshaft 30R. Specifically,an increase in the rotary speed of left axleshaft 30L caused by speedchanging gearset 58L causes a corresponding increase in the rotary speedof first side gear 70L which, in turn, causes pinions 72 to drive rightside gear 70R at a corresponding reduced speed. First mode clutch 60L isshifted between its released and locked modes via actuation ofpower-operated clutch actuator 96L in response to control signals fromECU 36. Specifically, first mode clutch 60L is operable in its releasedmode when clutch actuator 96L applies a predetermined minimum clutchengagement force on clutch pack 94L and is further operable in itslocked mode when clutch actuator 96L applies a predetermined maximumclutch engagement force on clutch pack 94L.

Second mode clutch 60R is shown to be operably disposed between ringgear 78R of second speed changing unit 58R and housing 52. Inparticular, second mode clutch 60R includes a clutch hub 90R that isfixed for rotation with ring gear 78R, a drum 92R non-rotatably fixed tohousing 52, a multi-plate clutch pack 94R operably disposed between hub90R and drum 92R, and a power-operated clutch actuator 96R. Second modeclutch 60R is operable in a first or “released” mode so as to permitunrestricted relative rotation of ring gear 78R. In contrast, secondmode clutch 60R is also operable in a second or “locked” mode to brakerotation of ring gear 78R, thereby causing the rotary speed of sun gear76R to be increased relative to carrier 68. Thus, second mode clutch 60Rfunctions in its locked mode to increase the rotary speed of rightaxleshaft 30R which, in turn, causes differential 56 to decrease therotary speed of left axleshaft 30L, thereby directing more drive torqueto right axleshaft 30R than is directed to left axleshaft 30L. Secondmode clutch 60R is shifted between its released and locked modes viaactuation of power-operated clutch actuator 96R in response to controlsignals from ECU 36. In particular, second mode clutch 60R operates inits released mode when clutch actuator 96R applies a predeterminedminimum clutch engagement force on clutch pack 94R while it operates inits locked mode when clutch actuator 96R applies a predetermined maximumclutch engagement force on clutch pack 94R.

As seen, power-operated clutch actuators 96L and 96R are shown inschematic fashion to cumulatively represent the components required toaccept a control signal from ECU 36 and generate a clutch engagementforce to be applied to corresponding clutch packs 94L and 94R. To thisend, FIG. 3 diagrammatically illustrates the basic components associatedwith such power-operated clutch actuators. Specifically, eachpower-operated actuator includes a controlled device 100, a forcegenerating mechanism 102, and a force apply mechanism 104. Inelectro-mechanical systems, controlled device 100 would represent suchcomponents as, for example, an electric motor or an electromagneticsolenoid assembly capable of receiving an electric control signal fromECU 36. The output of controlled device 100 would drive force generatingmechanism 102 which could include, for example, a ball ramp, a ballscrew, a leadscrew, a pivotal lever arm, rotatable cam plates, etc.,each of which is capable of converting the output of controlled device100 into a clutch engagement force. Finally, force apply mechanism 104functions to transmit and exert the clutch engagement force generated byforce generating mechanism 102 onto clutch packs 94L and 94R and caninclude, for example, an apply plate or a thrust plate. If ahydra-mechanical system is used, controlled device 100 could be anelectrically-operated control valve that is operable for controlling thedelivery of pressurized fluid from a fluid source to a piston chamber. Apiston disposed for movement in the piston chamber would act as forcegenerating mechanism 102. Preferably, controlled device 100 is capableof receiving variable electric control signals from ECU 36 forpermitting variable regulation of the magnitude of the clutch engagementforce generated and applied to the clutch packs so as to permit“adaptive” control of the mode clutches.

In accordance with the arrangement shown, torque distributing drivemechanism 28 is operable in coordination with yaw control system 34 toestablish at a least three distinct operational modes for controllingthe transfer of drive torque from input shaft 54 to axleshafts 30L and30R. In particular, a first operational mode is established when firstmode clutch 60L and second mode clutch 60R are both in their releasedmode such that differential 56 acts as an “open” differential so as topermit unrestricted speed differentiation with drive torque transmittedfrom carrier 68 to each axleshaft 30L and 30R based on the tractiveconditions at each corresponding rear wheel 32L and 32R.

A second operational mode is established when first mode clutch 60L isin its locked mode while second mode clutch 60R is in its released mode.As a result, left axleshaft 30L is overdriven by first speed changingunit 58L due to braking of ring gear 78L. As noted, such an increase inthe rotary speed of left axleshaft 30L causes a corresponding speeddecrease in right axleshaft 30R. Thus, this second operational modecauses right axleshaft 30R to be underdriven while left axleshaft 30L isoverdriven when such an unequal torque distribution is required toaccommodate the current tractive or steering condition detected and/oranticipated by ECU 36 and based on the particular control strategy used.Likewise, a third operational mode is established when first mode clutch60L is shifted into its released mode and second mode clutch 60R isshifted into its locked mode. As a result, right rear axleshaft 30R isoverdriven relative to carrier 68 by second speed changing unit 58Rwhich, in turn, causes left axleshaft 30L to be underdriven bydifferential 56 at a corresponding reduced speed. Accordingly, drivemechanism 28 can be controlled to function as both a limited slipdifferential and a torque vectoring device. For example, when left wheel32L losses traction, second mode clutch 60R can be actuated to send moredrive torque to right wheel 32R and reduce the speed of left wheel 32Lso as to equalize the wheel speeds. Alternatively, during a turn orcornering maneuver when more drive torque is needed at one wheel toreact to a yaw moment, the mode clutch associated with that wheel isactuated.

At the start of vehicle 10, power from engine 12 is transmitted to frontwheels 20L and 20R through transmission 14 and front differential 16.Drive torque is also transmitted to torque distributing drive mechanism28 through PTU 22 and propshaft 24 which, in turn, rotatably drivesinput pinion shaft 54. Typically, mode clutches 60L and 60R would benon-engaged such that drive torque is transmitted through differential56 to rear wheels 32L and 32R. However, upon detection of lost tractionat front wheels 20L and 20R, at least one of mode clutches 60L and 60Rcan be engaged to provide drive torque to rear wheels 32L and 32R basedon the tractive needs of the vehicles.

In addition to on-off control of mode clutches 60L and 60R to establishthe various drive modes associated with overdrive connections throughspeed changing units 58L and 58R, it is further contemplated andpreferred that variable clutch engagement forces can be generated bypower-operated actuators 96L and 96R to adaptively regulate theleft-to-right speed and torque characteristics. This “adaptive” controlfeature is desirable since it functions to provide enhanced yaw andstability control for vehicle 10. For example, a “reference” yaw ratecan be determined based on several factors including the steering angledetected by steering angle sensor 42, the speed of vehicle 10 ascalculated based on signals from the various speed sensors, and alateral acceleration as detected by lateral acceleration sensor 48. ECU36 compares this reference yaw rate with an “actual” yaw rate valuedetected by yaw sensor 46. This comparison will determine whethervehicle 10 is in an understeer or an oversteer condition, as well as theseverity of the condition, so as to permit yaw control system 34 to beadaptively control actuation of the mode clutches to accommodate suchsteering tendencies. ECU 36 can address such conditions by initiallyshifting drive mechanism 28 into one of the specific operational drivemode that is best suited to correct the actual or anticipated oversteeror understeer situation. Thereafter, variable control of the modeclutches permits adaptive regulation of the side-to-side torque transferand speed differentiation characteristics when one of the distinct drivemodes is not adequate to accommodate the current steer tractivecondition.

Referring now to FIG. 4, a modified version of drive mechanism 28 fromFIG. 2 is shown and designated by reference numeral 28A. As seen, alarge number of components are common to both drive mechanisms 28 and28A, with such components being identified by the same referencenumbers. However, in drive mechanism 28A, side gears 70L and 70R areshown to be integral with corresponding sun gears 76L and 76R. Inaddition, mode clutches 60L and 60R, which were disclosed to be of themulti-plate friction clutch variety, are replaced by first (left) andsecond (right) mode clutches, hereinafter referred to as first andsecond brake units 110L and 110R, respectively. Brake units 110L and110R are schematically shown to each include a band 112L and 112R offriction material that is bonded to ring gears 78L and 78R, and a brakeactuator 114L and 114R, respectively. Each brake actuator is apower-operated device that receives control signals from ECU 36 and ismoveable relative to its corresponding ring gear 78L and 78R so as topermit establishment of released and locked modes. Specifically, firstbrake unit 110L is operable in its released mode to permit unrestrictedrotation of ring gear 78L and in its locked mode to brake rotation ofring gear 78L. Likewise, second brake unit 110R is operable in itsreleased mode to permit unrestricted rotation of ring gear 78R and inits locked mode to brake rotation of ring gear 78R. Active yaw controlsystem 34 is again shown to be associated with drive mechanism 28A toselectively control actuation (i.e., on-off or adaptive) of brakeactuators 114L and 114R so as to vary the driven rotary speed ofaxleshafts 30L and 30R for controlling the side-to-side speeddifferentiation and torque transfer characteristics of drive mechanism28A.

Referring now to FIG. 5, another modified version of drive mechanism 28of FIG. 2 is shown and hereinafter referred to as drive mechanism 28B.Again, common components are identified with the same referencenumerals. In this embodiment, however, differential 56 has been movedoutboard of carrier 68 rather than the inboard arrangement shown in FIG.2. To accomplish this, side gear 70L is now shown to be fixed forrotation with ring gear 78L while side gear 70R is shown to be fixed forrotation with ring gear 78R. Pinions 72 are still rotatably mounted onpinion shafts 74 that couple ring gear 66 to carrier 68. Drive mechanism28B also works in conjunction with yaw control system 34 to establishthe three distinct operational modes. As before, with both mode clutchesreleased, differential 56 acts as an open differential with side gears70L and 70R driving corresponding ring gears 78L and 78R which, in turn,transfer drive torque to axleshafts 30L and 30R through speed changinggearsets 58L and 58R, respectively.

Drive mechanism 28B is also operable when first mode clutch 60L islocked and second mode clutch 60R is released to have first gearset 58Loverdrive left axieshaft 30L relative to ring gear 66 and carrier 68.Specifically, with ring gear 78L braked, side gear 70L is likewisebraked such that pinions 72 cause side gear 70R to be rotated at anincreased speed. This increased rotary speed of side gear 70R causescorresponding rotation of ring gear 78R which, in turn, causes sun gear76R to drive right axleshaft 30R at a reduced speed. In contrast, whenfirst mode clutch 60L is released and second mode clutch 60R is locked,second gearset 58R overdrives right axleshaft 30R due to braking of ringgear 78R. In addition, the concurrent braking of side gear 70R causes acorresponding increase in rotary speed of side gear 70L which, in turn,drives ring gear 78L so as to reduce the rotary speed of sun gear 70Land left axleshaft 30L.

Referring now to FIG. 6, a four-wheel drive vehicle 10′ is shownequipped with a power transfer unit 160 that is operable fortransferring drive torque from the output of transmission 14 to a first(i.e., front) output shaft 162 and a second (i.e., rear) output shaft164. Front output shaft 162 drives a front propshaft 166 which, in turn,drives front differential 16 for driving front wheels 20L and 20R.Likewise, rear output shaft 164 drives a rear propshaft 168 which, inturn, drives a rear differential 170 for driving rear wheels 32L and32R. Power transfer unit 160, otherwise known as a transfer case,includes a torque distributing drive mechanism 172 which functions totransmit drive torque from its input shaft 174 to both of output shafts162 and 164 so as to bias the torque distribution ratio therebetween,thereby controlling the tractive operation of vehicle 10′. As seen,torque distribution mechanism 172 is operably associated with a tractioncontrol system 34′ for providing this adaptive traction control featurefor vehicle 10′.

Referring primarily to FIG. 7, torque distribution mechanism 172 ofpower transfer unit 160 is shown to be generally similar in structure todrive mechanism 28 of FIG. 2 with the exception that carrier 68 is nowdrivingly connected to input shaft 174 via a transfer assembly 180. Inthe arrangement shown, transfer assembly 180 includes a first sprocket182 driven by input shaft 174, a second sprocket 184 driving carrier 68,and a power chain 186 therebetween. As seen, differential 56 now acts asa center or “interaxle” differential for permitting speeddifferentiation between the front and rear output shafts whileestablishing a full-time four-wheel drive mode. In particular, frontoutput shaft 162 is fixed for rotation with side gear 70L ofdifferential 56 and sun gear 76L of speed changing unit 58L. Likewise,rear output shaft 164 is fixed for rotation with side gear 70R ofdifferential 56 and sun gear 76R of speed changing unit 58R. As seen,first mode clutch 60L is still arranged to control braking of ring gear78L while second mode clutch 60R is arranged to control braking of ringgear 78R.

Controlled actuation of mode clutches 60L and 60R results incorresponding increases or decreases in the rotary speed of rear outputshaft 164 relative to front output shaft 162, thereby controlling theamount of drive torque transmitted therebetween. In particular, whenboth mode clutches are released, unrestricted speed differentiation ispermitted between the front and rear output shafts while the gear ratioestablished by the components of interaxle differential 56 controls thefront-to-rear torque ratio based on the current tractive conditions ofthe front and rear wheels. An adaptive full-time four-wheel drive modeis made available via traction control system 34′ to limit interaxleslip and vary the front-rear drive torque distribution ratio based onthe tractive needs of the front and rear wheels as detected by thevarious sensors. In addition to power transfer unit 160, vehicle 10′could also be equipped with a rear axle assembly having either torquedistributing drive mechanism 28 or 28′ and its corresponding yaw controlsystem, as is identified by the phantom lines in FIG. 6.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A motor vehicle, comprising: a powertrain operable to generate drivetorque; a first driveline having a first shaft driving a pair of firstwheels; a second driveline having a second shaft driving a pair ofsecond wheels; a power transfer unit for transmitting drive torque fromsaid powertrain to said first and second shafts, said power transferunit including a differential, first and second planetary gearsets andfirst and second mode clutches, said differential having an inputcomponent driven by said powertrain and first and second outputcomponents driving said first and second shafts, said first planetarygearset having a first gear element coupled for rotation with said firstoutput component, a second gear element and a third gear element inconstant mesh with said first and second gear elements, said secondplanetary gearset having a fourth gear element coupled for rotation withsaid second output component, a fifth gear element and a sixth gearelement in constant mesh with said fourth and fifth gear elements, saidfirst mode clutch is operable for retarding rotation of said second gearelement so as to increase the rotary speed of said first gear element,and said second mode clutch is operable for retarding rotation of saidfifth gear element so as to increase the rotary speed of said fourthgear element; and a control system for controlling actuation of saidfirst and second mode clutches.
 2. The motor vehicle of claim 1 whereinsaid power transfer unit is operable to establish a first four-wheeldrive mode when said first mode clutch is engaged and said second modeclutch is released, whereby said first output component is overdrivenrelative to said input component such that said differential causes saidsecond output component to be underdriven relative to said inputcomponent.
 3. The motor vehicle of claim 2 wherein said power transferunit is operable to establish a second four-wheel drive mode when saidfirst mode clutch is released and said second mode clutch is engaged,whereby said second output component is overdriven relative to saidinput component such that said differential causes said first outputcomponent to be underdriven relative to said input component.
 4. Themotor vehicle of claim 1 wherein said differential includes a carrier asits input component, a first side gear as its first output component, asecond side gear as its second output component and pinion gearssupported by said carrier and which are meshed with said first andsecond side gears.
 5. The motor vehicle of claim 4 wherein said thirdand sixth gear elements are rotatably supported on said carrier.
 6. Themotor vehicle of claim 4 wherein said first gear element is a first sungear coupled for rotation with said first side gear, said second gearelement is a first ring gear and said third gear element is a firstplanet gear meshed with said first sun gear and said first ring gear,and wherein said fourth gear element is a second sun gear coupled forrotation with said second side gear, said fifth gear element is a secondring gear and said sixth gear element is a second planet gear meshedwith said second sun gear and said second ring gear.
 7. The motorvehicle of claim 6 wherein said first mode clutch is disposed betweensaid first ring gear and a stationary member and includes a firstpower-operated clutch actuator operable to generate and exert a clutchengagement force on said first mode clutch, wherein said second modeclutch is disposed between said second ring gear and said stationarymember and includes a second power-operated clutch actuator operable togenerate and exert a clutch engagement force on said second mode clutch,and wherein said control system includes a control unit operable tocontrol actuation of said first and second clutch actuators.
 8. Themotor vehicle of claim 1 wherein said first mode clutch is disposedbetween said second gear element and a stationary member and includes afirst clutch actuator operable to engage said first mode clutch, whereinsaid second mode clutch is disposed between said fifth gear element andsaid stationary member and includes a second clutch actuator operable toengage said second mode clutch, and wherein said control unit isoperable to control actuation of said first and second clutch actuators.9. A motor vehicle, comprising: a powertrain operable for generatingdrive torque; a first driveline having a first shaft driving a pair offirst wheels; a second driveline having a second shaft driving a pair ofsecond wheels; a power transfer unit including a differential, first andsecond speed changing units and first and second mode clutches, saiddifferential having an input component driven by said powertrain andfirst and second output components, said first speed changing unitincluding a first gear driving said first shaft, a second gear driven bysaid first output component and a third gear meshed with said first gearand said second gear, said second speed changing unit including a fourthgear driving said second shaft, a fifth gear driven by said secondoutput component and a sixth gear meshed with said fourth gear and saidfifth gear, said first mode clutch is operable for reducing the rotaryspeed of said second gear for increasing the rotary speed of said firstshaft, and said second mode clutch is operable for reducing the rotaryspeed of said fifth gear for increasing the rotary speed of said secondshaft; and a control system for controlling actuation of said first andsecond mode clutches.
 10. The motor vehicle of claim 9 wherein saidpower transfer unit is operable to establish a first four-wheel drivemode when said first mode clutch is engaged and said second mode clutchis released, whereby said first shaft is overdriven relative to saidinput component such that said differential causes said second shaft tobe underdriven relative to said input component.
 11. The motor vehicleof claim 10 wherein said power transfer unit is operable to establish asecond four-wheel drive mode when said first mode clutch is released andsaid second mode clutch is engaged, whereby said second shaft isoverdriven relative to said input component such that said differentialcauses said first shaft to be underdriven relative to said inputcomponent.
 12. The motor vehicle of claim 9 wherein said differentialincludes a carrier as its input component, a first side gear as itsfirst output component, a second side gear as its second outputcomponent and pinion gears supported by said carrier and which aremeshed with said first and second side gears.
 13. The motor vehicle ofclaim 12 wherein said third and sixth gears are rotatably supported onsaid carrier, wherein said first side gear is connected for commonrotation with said second gear, and wherein said second side gear isconnected for common rotation with said fifth gear.
 14. The motorvehicle of claim 9 wherein said first mode clutch is disposed betweensaid second gear and a stationary member and includes a firstpower-operated clutch actuator operable to engage said first modeclutch, wherein said second mode clutch is disposed between said fifthgear and said stationary member and includes a second power-operatedclutch actuator operable to engage said second mode clutch, and whereinsaid control system includes a control unit operable to controlactuation of said first and second clutch actuators.
 15. A powertransfer unit for use in a motor vehicle having a powertrain and firstand second drivelines comprising: a first shaft driving the firstdriveline; a second shaft driving the second driveline; a differentialhaving an input component driven by the powertrain, a first outputcomponent driving said first shaft and a second output component drivingsaid second shaft; a first speed changing unit having a first geardriven by said first output component and a second gear; a second speedchanging unit having a third gear driven by said second output componentand a fourth gear; a first mode clutch operable for braking rotation ofsaid second gear; a second mode clutch operable for braking rotation ofsaid fourth gear; and a control system for controlling actuation of saidfirst and second mode clutches, wherein a first drive mode isestablished when said first mode clutch is engaged and said second modeclutch is released, whereby said first shaft is overdriven relative tosaid input component and said differential causes said second shaft tobe underdriven relative to said input component.
 16. The power transferunit of claim 15 wherein a second drive mode is established when saidfirst mode clutch is released and said second mode clutch is engaged,whereby said second shaft is overdriven relative to said input componentand said differential causes said first shaft to be underdriven relativeto said input component.
 17. The drive axle assembly of claim 15 whereinsaid differential includes a carrier as its input component, a firstside gear as its first output component, a second side gear as itssecond output component and pinion gears supported by said carrier andwhich are meshed with said first and second side gears.
 18. The driveaxle assembly of claim 17 wherein said first speed changing unit furtherincludes a fifth gear meshed with said first and second gears, whereinsaid second speed changing unit further includes a sixth gear meshedwith said third and fourth gears, and wherein said fifth and sixth gearsare rotatably supported on said carrier.
 19. The drive axle assembly ofclaim 15 wherein said first mode clutch is disposed between said secondgear and a stationary member and includes a first clutch actuatoroperable to engage said first mode clutch, wherein said second modeclutch is disposed between said fourth gear and said stationary memberand includes a second clutch actuator operable to engage said secondmode clutch, and wherein said control system is operable to controlactuation of said first and second clutch actuators.
 20. A powertransfer unit for use in a motor vehicle having a powertrain and firstand second drivelines, comprising: an input shaft driven by thepowertrain; a first shaft driving the first driveline; a second shaftdriving the second driveline; a differential having an input componentdriven by said input shaft; a first output component driving said firstshaft and a second output component driving said second shaft; a firstspeed changing unit having a first sun gear fixed for rotation with saidfirst shaft, a first ring gear and a set of first planet gears meshedwith said first sun gear and said first ring gear; a second speedchanging unit having a second sun gear fixed for rotation with saidsecond shaft, a second ring gear and a set of second planet gears meshedwith said second sun gear and said second ring gear; a first mode clutchoperable for braking rotation of said first ring gear; a second modeclutch operable for braking rotation of said second ring gear; and acontrol system for controlling actuation of said first and second modeclutches.
 21. The power transfer unit of claim 20 wherein said firstmode clutch is disposed between said first ring gear and a stationarymember and includes a first clutch actuator operable to engage saidfirst mode clutch, wherein said second mode clutch is disposed betweensaid second ring gear and said stationary member and includes a secondclutch actuator operable to engage said second mode clutch, and whereinsaid control system is operable to control actuation of said first andsecond clutch actuators.
 22. A motor vehicle, comprising: a powertrainoperable for generating drive torque; a first driveline having a firstshaft driving a pair of first wheels; a second driveline having a secondshaft driving a pair of second wheels; a power transfer unit forselectively transmitting drive torque from said powertrain to said firstand second shafts, said power transfer unit including a differential,first and second speed changing units and first and second modeclutches, said differential having an input component driven by saidpowertrain, a first output component driving said first shaft and asecond output component driving said second shaft, said first speedchanging unit having a first sun gear driven by said first outputcomponent, a first ring gear and a set of first planet gears meshed withsaid first sun gear and said first ring gear, said second speed changingunit having a second sun gear driven by said second output component, asecond ring gear and a set of second planet gears meshed with saidsecond sun gear and said second ring gear, said first mode clutch isoperable for braking rotation of said first ring gear and said secondmode clutch is operable for braking rotation of said second ring gear;and a control system for controlling actuation of said first and secondmode clutches.
 23. The motor vehicle of claim 22 wherein said powertransfer unit is operable to establish a first drive mode when saidfirst mode clutch is engaged and said second mode clutch is released,whereby said first shaft is overdriven relative to said input componentand said differential causes said second shaft to be underdrivenrelative to said input component.
 24. The motor vehicle of claim 23wherein said power transfer unit is operable to establish a second drivemode when said first mode clutch is released and said second mode clutchis engaged, whereby said second shaft is overdriven relative to saidinput component and said differential causes said first shaft to beunderdriven relative to said input component.